I ran stock bottom end 340s 7800 RPM for years with no issues other than pulling the big end of the rod egg shaped a little bit. When I went to stock 360 iron cranks is when the problems started. The first was an iron 3.58 crank deal in a 340 block that spun a rod and blew up in 6 runs. The second was an iron 3.58 crank 360 block 7400 RPM deal that had to have 1 or 2 rod bearings replaced every 6 or 7 runs. Same rods everytime. You see copper in the System 1, you take the pan off and fix it. HV pump, good 10w30 oil, plenty of clearance, everything else looked fine. Tried oil pans, cross drilling and a bunch of other stuff which made zero difference. Ended up completely redoing the oil system in the block with all the tricks and even made up a few of my own. Would run all season and be fine after that and that's with thin oil and oil pressure numbers that would make most people pucker up.

Usually you see these problems with the 360s pick on one rod, maybe 2 once they try to spin it to 7k+. In your case, it sounds like all the bearings were hot and #8 burnt up first. The oil for 8 comes right off the oil filter, not the galley. It should be the last thing to run dry. That passage also oils the rear cam bearing. What's it look like?

Despite what the oil pressure gauge said, the problem almost has to be pump, thick oil through a tight filter, adapter, passages or something else in the rear of the block related, for the entire bottom end to look like it was starved for oil.